I. Field of the Invention
This invention relates to a process for treating caustic cyanide and metal containing wastes which have a tendency to form an insoluble scale on heating.
II. Description of Related Art
Caustic cyanide wastes containing metals are generated by the metal plating industry. The wastes are hazardous for both their cyanide and metals content. These wastes have been treated by alkaline chlorination to destroy cyanide but this treatment generally results in generation of solids which still contain appreciable amounts of cyanide. See for example Jahnsen U.S. Pat. No. 4,059,514.
Wet air oxidation provides an alternative method for cyanide destruction and waste treatment. The elevated temperature and pressure oxidation provides a high degree of cyanide destruction for these wastes. The high solids content of these caustic cyanide and metal wastes gives rise to scaling problems in wet air oxidation systems which can require frequent shut down to remove scale from the system. Scaling is a problem in feed lines, within the wet oxidation reactor, and in oxidized effluent lines when these wastes are treated by wet air oxidation.
Numerous wet air oxidation flow schemes are known but none of them were judged to be applicable to the wet oxidation treatment of such caustic cyanide and metal wastes.
Morgan in U.S. Pat. No. 3,849,536 and U.S. Pat. No. 3,917,460 describe the wet oxidation of soda process pulping liquor to oxidize the organic constituents therein. The wet oxidation reactor is divided into upper and lower reaction zones with preheated pulping liquor and air introduced into the lower zone where the bulk of the oxidation occurs. The oxidation mixture moves to the top of the upper zone where offgases are vented and the liquor travels downward encountering additional air introduced at the bottom of the upper zone. The highly oxidized liquor exits near the bottom of the upper zone.
Tagashira, U.S. Pat. No. 4,070,281 discloses the wet oxidation of cyanogen containing waste using a combination of copper and ammonium salts. Some post wet oxidation treatment is required to remove residual cyanide in the wet oxidation waste. No specific continuous flow scheme is mentioned.
Barr in U.S. Pat. No. 4,108,680 discloses a process for removing calcium oxalate scale from metal surfaces using a mixture of dilute nitric acid and manganese dioxide.
Pradt et al. in U.S. Pat. No. 4,174,280 describes the wet oxdation of liquid or solid combustible materials which are insoluble, immiscible and difficult to suspend in water. Water and air are preheated and introduced into the oxidation reactor. A suspension of waste is also injected directly into the reactor where oxidation occurs. The oxidized effluent and offgases are removed from the reactor through a heat exchanger used to preheat the incoming water and air.
Chowdhury in U.S. Pat. No. 4,350,599 discloses wet oxidation of caustic waste liquor where preheated waste and air or oxygen react to generate carbon dioxide which is used to reduce the corrosive nature of the raw waste.
Okugawa in U.S. Pat. No. 4,402,837 describes a process for heat treatment of nickel cyanide waste with caustic and steam using conventional heat exchangers and a horizontal reactor with one outlet for treated liquid and another for ammonia and steam removal.
Horak et al. in U.S. Pat. No. 4,525,283 describes a catalytic wet oxidation of waste at acid pH. A series of wet oxidation reactors is shown where preheated waste and oxygen gas enter the bottom of the reactors and the top of the reactors is provided with an effluent overflow line plus gas transfer, gas circulating and waste gas pipes.
The present invention provides a process for the continuous wet air oxidation of caustic cyanide and metal wastes which provides for uninterrupted waste treatment. The problem of excessive scaling is overcome resulting in the continuous production of an effluent with only trace amounts of cyanide remaining therein.